1. Field of the Invention
The present invention relates to a measuring chip that is employed in a surface plasmon resonance measuring apparatus for quantitatively analyzing a substance in a sample by utilizing the excitation of a surface plasmon. The present invention also relates to a method of manufacture of a measuring chip as described above.
2. Description of the Related Art
In metals, if free electrons are caused to vibrate in a group, compression waves called plasma waves will be generated. The compression waves generated in a metal surface and quantized are called surface plasmon.
A variety of surface plasmon resonance measuring apparatuses have been proposed to quantitatively analyze a substance in a sample by taking advantage of a phenomenon that surface plasmon is exited by light waves. Among the apparatuses, an apparatus employing a system called “Kretschmann configuration” is particularly well known (e.g., see Japanese Unexamined Patent Publication No. 6 (1994)-167443).
The surface plasmon resonance measuring apparatus employing the “Kretschmann configuration” is equipped mainly with a dielectric block formed, for example, into the shape of a prism; a metal film, formed on a surface of the dielectric block, for placing a sample thereon; a light source for emitting a light beam; an optical system for making the light beam enter the dielectric block so that a condition for total internal reflection is satisfied at the interface between the dielectric block and the metal film and that various angles of incidence, including a surface plasmon resonance condition, are obtained; and photodetection means for measuring the intensity of the light beam satisfying total internal reflection at the interface to detect surface plasmon resonance.
In order to obtain various angles of incidence in the aforementioned manner, a relatively thin light beam may be caused to strike the above-mentioned interface at different angles of incidence, or relatively thick convergent or divergent rays may be caused to strike the interface so that they contain components incident at various angles. In the former, a light beam whose reflection angle varies with deflection of the light beam, can be detected by a small photodetector that is moved in synchronization with the light beam deflection, or by an area sensor extending in the direction where the angle of reflection varies. In the latter, on the other hand, rays reflected at various angles can be detected by an area sensor extending in a direction where all the reflected rays can be received.
In the surface plasmon resonance measuring apparatus mentioned above, if a light beam strikes the metal film at a specific incidence angle θsp equal to or greater than a critical angle of incidence at which total internal reflection takes place, evanescent waves having electric field distribution are generated in the sample in contact with the metal film, whereby surface plasmon is excited at the interface between the metal film and the sample. When the wave vector of the evanescent light is equal to the wave number of the surface plasmon and therefore the wave numbers between the two are matched, the evanescent waves and the surface plasmon resonate and light energy is transferred to the surface plasmon, whereby the intensity of light satisfying total internal reflection at the interface between the dielectric block and the metal film drops sharply. The sharp intensity drop is generally detected as a dark line by the above-mentioned photodetection means.
Note that the above-mentioned resonance occurs only when the incident light beam is a p-polarized light beam. Therefore, in order to make the resonance occur, it is necessary that a light beam be p-polarized before it strikes the interface.
If the wave number of the surface plasmon is found from a specific incidence angle θsp at which attenuated total reflection (hereinafter referred to as ATR) takes place, the dielectric constant of a sample to be analyzed can be calculated by the following Equation:Ksp(ω)=(ω/C){∈m(ω)∈s}1/2/{∈m(ω)+∈s}1/2 where Ksp represents the wave number of the surface plasmon, ω represents the angular frequency of the surface plasmon, c represents the speed of light in vacuum, and ∈m, and ∈s represent the dielectric constants of the metal and the sample, respectively.
If the dielectric constant ∈s of the sample is found, the density of a specific substance in the sample is found based on a predetermined calibration curve, etc. As a result, the specific substance can be quantitatively analyzed by finding the incidence angle θsp at which the intensity of reflected light drops.
In the conventional surface plasmon resonance measuring apparatus employing the aforementioned system, the metal film on which a sample is placed must be exchanged for a new one each time a measurement is made. Because of this, the metal film is fixed to a first dielectric block in the form of a plate, and a second dielectric block in the form of a prism is provided as an optical coupler for making the aforementioned total internal reflection occur. The first dielectric block is united with a surface of the second dielectric block. The second dielectric block is fixed with respect to an optical system, and the first dielectric block and the metal film are used as a measuring chip. In this manner, the measuring chip can be exchanged for a new one, every time a measurement is made.
However, in the case where the conventional exchangeable measuring chip is employed, a gap occurs between the first dielectric block and the second dielectric block and the refractive index becomes discontinuous. To prevent the discontinuity, it is necessary that the two dielectric blocks be united through an index-matching solution. The operation of uniting the two dielectric blocks in a body is fairly difficult, and consequently, the conventional measuring chip is not easy to handle in making a measurement. Particularly, in the case where measurement is automated by automatically loading a measuring chip into a turret, then rotating the turret, and automatically supplying the measuring chip to a measuring position where a light beam enters the measuring chip, the loading and removal of the measuring chip is time-consuming, resulting in a reduction in the efficiency of the automatic measurement.
In addition, there is a possibility that the conventional measuring chip will have a detrimental influence on the environment, because it uses an index-matching solution.
In view of the circumstances mentioned above, the applicant has proposed a surface plasmon resonance measuring chip that can be easily exchanged for a new one without requiring an index-matching solution (Japanese Unexamined Patent Publication No. 2000-212125).
This surface plasmon resonance measuring chip is equipped with a dielectric block; a metal film, formed on a surface of the dielectric block, for placing a sample thereon; a light source for emitting a light beam; an optical system for making the light beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection is satisfied at an interface between the dielectric block and the metal film; and photodetection means for detecting the intensity of the light beam satisfying total internal reflection at the interface to detect surface plasmon resonance. The dielectric block is formed as a single block that includes an entrance surface which the light beam enters, an exit surface from which the light beam emerges, and a surface on which the metal film is formed. The metal film is united with the dielectric block.
In the surface plasmon resonance measuring chip disclosed in the aforementioned publication No. 2000-212125, the dielectric block is formed as a single block that includes an entrance surface which the light beam enters, an exit surface from which the light beam emerges, and a surface on which the metal film is formed (this block also functions as an optical coupler because it includes an entrance surface and an exit surface), and the dielectric block is united with the metal film. Therefore, if only the measuring chip is installed in and removed from the optical system, the measuring chip can be easily exchanged for a new one.
That is, since the surface plasmon resonance measuring chip does not require the aforementioned two dielectric blocks, the measuring chip does not have to employ an index-matching solution through which the two dielectric blocks are united. Thus, the measuring chip is capable of eliminating the inconvenience of handling that is caused by employing an index-matching solution.
In addition, if the measuring chip does not need to employ an index-matching solution, the measuring chip is prevented from having a detrimental influence on the environment.
Note that desirable materials for the dielectric block are glass and synthetic resin. Particularly, synthetic resin is advantageous in that measuring chips can be manufactured at low costs by injection molding.
However, in the case where measuring chips are formed from synthetic resin, the problem of a reduction in the signal-to-noise (S/N) ratio for the output signal of the photodetection means that detects surface plasmon resonance will arise.